Polycarbonate polymers are highly regarded engineering resins because of their excellent physical properties. For example, aromatic polycarbonates formed from dihydric phenols exhibit high tensile and impact strength as well as very good thermal stability.
Blow molding is a well known process for making hollow objects. In general, the process consists of extruding a molten tube of resin from a die to an open mold, which is then closed around the tube (parison), the bottom of tube having been pinched together by the mold. Air and pressure is fed through the die into the tube which expands to fill the mold, and the part is cooled as it is held under internal air pressure. The cooled part falls free on opening the mold.
It is generally recognized that a resin for use in blow molding should have certain consistent processing properties. In particular, the parison melt should be resistant to sag. If a large object is to be made, the molding equipment can be provided with a cylinder and a piston as an accumulator for the melt. The melt from the extruder is fed to the accumulator, from which it is emptied at a much faster rate to form a large parison, thereby minimizing sag of the molten tube. Ideally, to reduce sag the molten resin should show a strong dependence on shear rate of the tactoscopic variety, i.e., it should exhibit increasing viscosity as shear rate decreases, and this property should manifest itself at low shear rates. Resistance to sag of the material between the die and the rollers is also of some importance in extruding sheet stock.
It is an object of this invention to provide a polycarbonate resin blend having improved rheological properties for blow molding.
It is a further object of this invention to provide a resin blend in which the desirable physical properties of the aromatic polycarbonate resin is preserved.
It is a still further object of this invention to provide an improved process for blow molding polycarbonate resins.
These and other objects will become evident to one skilled in the art on reading this entire specification including the appended claims.